The amounts of energy stored and charge rate of the store is obtained from the model and used for the analysis. The charge rate is expressed as;
( ) w p in out m C T T 10.36
10.3 Simulation result and analysis and Validation of model (PHE)
The Matlab model was used to validate the experiment result done using RT 58 and for RT 52; at different flow rates (low, medium and high flow). The Matlab code use for the validation of the experiment for both charging and discharging is presented in Appendix J. Figure 10:4 shows the result for the validation of the model using RT 58 experimental result. Figure 10:5 to Figure 10:7 shows the temperature profile of the inlet heat transfer fluid and outlet HTF during experiment and the outlet HTF temperature from the simulation using the MATLAB model for different flows using RT 52 data. The simulation result for the discharging process of RT 52 is presented
using Figure 10:8 and Figure 10:9. The inlet HTF temperature during the experiment was used in the simulation to compare the simulation HTF outlet temperature with measured experimental values. This was done to validate the heat transfer model for the system written in MATLAB utilising material properties from the DSC and hot disk instrument, along with properties of polypropylene (PP) sheet from literature. The results show that the outlet temperature predicted by the simulation is in accordance with the experimentally measured data, which means the model functions appropriately. The validated model is used to study the effects of varying the relevant parameters during charging and discharging.
Figure 10:4: Temperature profile of validated model for RT 58 experiment. The model was initially used to validate the charging result using RT 58 as shown in Figure 10:4, where the experiment was done at high flow only. With modification of the flow process to facilitate varying flow rates, a valve was installed before the commencement of the charging process using RT 52. The model was also used to validate experiment done using RT 52, which has different thermal properties (specific heat for when the PCM is solid and liquid, latent heat, melting point etc.), different flow rate (low, medium and high flow) and at an HTF inlet temperature of 60°C. Figure 10:5 to Figure 10:7 shows that the model was appropriate for the three different flow rates using RT 52 as the PCM for the thermal store. There is a noticeable difference between the outlet HTF temperature and the simulation outlet temperature at the beginning of the simulation for Figure 10:4 to Figure 10:7.
0 100 200 300 400 500 600 700 800 900 20 25 30 35 40 45 50 55 60 65 T e m p e ra tu re (° C )
Time in tens of seconds
Temperature profile of thermal store (High flow) RT 58
Outlet HTF experiment Inlet HTF experiment Outlet HTF simulation
Figure 10:5: Temperature profile of the validated model for RT 52 at low flow rate.
Figure 10:6: Temperature profile of the validated model for RT 52 at medium flow. 0 200 400 600 800 1000 1200 1400 1600 1800 2000 15 20 25 30 35 40 45 50 55 60 T e m p e ra tu re (° C )
Time in tens of seconds
Temperature profile of thermal store (Low flow) RT 52
Outlet HTF experiment Inlet HTF experiment Outlet HTF simulation 0 200 400 600 800 1000 1200 1400 1600 1800 2000 20 25 30 35 40 45 50 55 60 T e m p e ra tu re (° C )
Time in tens of seconds
Temperature profile of thermal store (Medium flow) RT 52
Outlet HTF experiment Inlet HTF experiment Outlet HTF simulation
Figure 10:7: Temperature profile of the validated model for RT 52 at high flow.
For the validation of the discharge process based on experimental data using RT 52, the low and medium flows are presented for the discharge at different HTF temperatures; 38°C and 47°C respectively. The discharge process at high flow is not presented for the discharge was done, without the use of the hot water bath. Figure 10:8 and Figure 10:9 shows the result from the simulation, where the experimental inlet HTF temperature and outlet HTF temperature are plotted alongside the simulation outlet HTF temperature. The model does not exactly agree with the experiment results throughout the duration of the simulation. This can be attributed to the nature of the solidification process, where temperature variations could occur as a result of PCM forming layers on the heat transfer surface.
0 200 400 600 800 1000 1200 1400 1600 1800 2000 30 35 40 45 50 55 60 T e m p e ra tu re (° C )
Time in tens of seconds
Temperature profile of thermal store (High flow) RT 52
Outlet HTF experiment Inlet HTF experiment Outlet HTF simulation
Figure 10:8: Temperature profile of the validated model for RT 52 at low flow (Discharge Process).
Figure 10:9: Temperature profile of the validated model for RT 52 at medium flow (Discharge process).
. 0 500 1000 1500 2000 2500 3000 3500 4000 35 40 45 50 55 60
Time in tens of seconds
T e m p e ra tu re (° C )
Temperature profile Discharging Process (Low flow) RT 52
Outlet HTF experiment Inlet HTF experiment Outlet HTF simulation 0 100 200 300 400 500 600 700 800 900 1000 45 50 55 60
Time in tens of seconds
T e m p e ra tu re (° C )
Temperature profile of Discharging Process (Medium flow) RT 52
Outlet HTF experiment Inlet HTF experiment Outlet HTF simulation
10.3.1 Varying the flow rate
Using the model, the properties of the sheet was varied for the RT 52 and RT 58. The simulation results obtained using RT 58 is presented in this section of the thesis, since the model works well for both cases. Various parameters such as thickness of PCM, latent heat, mass flow rate is discussed in this section. The mass flow rate is varied to study the effect it has on the amount of energy stored and thermal behaviour of the rig. From the result shown in Figure 10:10, it is observed that varying the mass flow rate has a significant effect on the temperature profile of the model. However, it has no effect on the amount of energy store profile; Figure 10:11 shows the energy profile. This agrees with findings from (Mohamed (2011)),which states that the heat transfer fluid mass flow rate has no significant effect on the accumulated stored energy. It is observed that when the mass flow rate is increased by a factor of 2 (130g/s), the simulation HTF outlet temperature trend (green line) moves upwards; away from the experimental result. However, reducing the mass flow by the same factor (32.5g/s), the simulation outlet temperature trend (green line) moves downwards as shown in Figure 10:10. The mass flow rate affects the melting time of the rig, because the rate of heat transfer increases with increase in the mass flow rate. However, increasing the mass flow rate does not increase the performance of the thermal store with regards the amount of energy stored. Based on the results from the experiment and the simulation using the MATLAB code, it is observed that it takes a longer time for the PCM to melt at low flow compared to the medium and high flow. This means the flow rate can be adjusted to suite the desired period of charging/discharging, depending on how much time the store is required to be charged during the off peak periods and how often charging and discharging is required throughout the day.
a) 32.5g/s b )130g/s
Figure 10:10: Model result increasing and reducing the flow rate.
Figure 10:11: Energy stored against time for RT 58.
10.3.2 Varying the thickness of the PCM
The thickness of the PCM is varied using the Matlab code to study the effect on the thermal store performance. The thickness of PCM used for the experiment and to validate the model is 10mm. The original thickness of the PCM was halved (5mm) and quadrupled (40mm). Figure 10:12 shows the result of the simulation. There is an
0 100 200 300 400 500 600 700 800 900 1000 -1 0 1 2 3 4 5 6 7 8 9x 10 6
Time in tens of seconds
J o u le s s to re d
increase in the amount of energy stored when the PCM thickness is increased by four times (quadrupled) the original value as shown in Figure 10:12 a). However, there is no change in the temperature profile. This was observed also, when the thickness of the PCM was halved. At half of the original thickness of the PCM, the amount of energy obtainable from the thermal store was less when compared to the original thickness of the PCM(see Figure 10:12 b)). The volume of PCM will result in an increase in the amount of energy obtainable from the thermal store. The result shows that when the thickness of PCM is increased by four times the original value, there is a corresponding increase in the energy obtainable from the thermal store, when compared to amount of energy stored when the thickness of PCM is halved. This means that the PHE thermal store output can be varied by changing the thickness. However, it was observed that the time it takes to run the simulation increases as the thickness of the PCM is increased; this is as a result of more simulation time required to melt the layers of PCM with respect to the thickness of the PCM in the thermal store.
a)40mm thickness b)5mm thickness
Figure 10:12: Effect on varying the thickness of the PCM.
10.3.3 Varying latent heat of PCM used for experiment.
The latent heat of the PCM was doubled and halved to find out the effect on the performance of the store. From the DSC result for RT 58, the value of the latent heat
the thermal store using the model. Figure 10:13 b) shows the energy of the store rose up to 10MJ using a PCM with a latent heat of 280kJ/kg, while using a PCM with a latent heat of 70kJ/kg (see Figure 10:13 a)) resulted in about 8MJ of energy from the thermal store. The same result experienced with varying the thickness of the PCM was applicable to when the latent heat of the PCM was varied. Using a PCM with a higher latent heat, it would result in a higher amount of energy from the store, though other factors that determine selection of PCM needs to be considered. The temperature profile of the model was also not affected by varying the latent heat of the PCM in the simulation.
a)Latent heat (70kJ/kg) b)Latent heat(280kJ/kg)
Figure 10:13: Effect of varying the latent heat of the PCM.